Conventional practice in preparing paper making stock is to feed a suspension of liquid and paper making fibers, which contains in varying degrees undesirable rejects, through a screen to remove at least a portion of the rejects before the stock is delivered to the forming surface of a paper making machine.
One popular type of screening apparatus utilizes a perforated, cylindrical screen into the interior of which is fed the unscreened paper making stock. Rejects are withdrawn from an end, usually the lower end, of the vertically oriented screen, while the accepts are passed through the perforations in the screen and collected. Additionally, rotating foils or other devices are positioned either inside or outside the screen surface, generally to alleviate plugging of the screen holes.
For example, in U.S. Pat. No. 3,617,008, a screen is disclosed which may have round holes or slots formed therethrough with the slots either horizontally or vertically oriented. A device such as a foil rotates over the inner or outer surface of the screen and flow impediments cooperate with the foils to cut off flow movement parallel to the surface of the screen and in some embodiments, to actually cooperate with the foils to sever filament shaped impurities in the product being screened.
While the size of the screen openings contemplated are not disclosed it may be assumed that the slots are, as is conventional, on the order of something in excess of 0.010 inch in width and thus, a substantial amount of the undesirable rejects below this size will pass through the screen with the accepts. Additionally, it is again not specifically disclosed but it may be assumed that, as in the case of conventional slotted cylindrical screens, a much higher than desirable removal of paper making fibers with the rejects would occur if an attempt were made to remove very small size rejects.
In U.S. Pat. No. 3,849,302, a commercially successful method and apparatus for screening paper making stock is disclosed. The apparatus includes a circular screen having vertically oriented slots and rotatable foils which move past the inner face of the cylindrical screen but are spaced a sufficient distance from the screen inner face to establish a tubular layer of stock adjacent the screen inner face. This provides improved screening results as compared to more conventional screens in which round holes are provided and, although it is stated in this patent that the vertical slot widths can be as low as 0.003 to 0.006 inch, it is also stated that as a practical and economic matter the slot widths should be in the range of approximately 0.010 to 0.030 inch.
Additionally, it has been found in subsequent development work that with vertical slots having a width much lower than 0.010 inch, only "fines" are accepted and an appreciable amount of the more desirable, longer, paper making fibers are lost along with the rejects. Aside from the loss of good fibers the use of narrower vertical slots in commercially available screens of this type results in a marked decrease in throughput rates since the percentage of accepts is necessarily relatively low as compared to the quantity of feed.
In order to circumvent the problems connected with extremely small perforations and still prevent small particles from passing through the openings, conventional screens have been designed to produce auxiliary screening effects. These may be classified into two groups. The first group claims to form a layer network of long fibers and reject particles on the inlet side of the screen cylinder. The openings of the network are claimed to be very small and represent the actual criteria which determines which particles are accepted or rejected rather than the size of the openings through the screen itself.
The second group of screens is based on the hypothesis that such a layer is ineffective and that the acceptance or rejection of a particle is determined by the size and shape of the opening in the screen cylinder. Designs are aimed at orienting elongated particles in such a way that the longest or broadest sides of the particles are presented to the opening -- resulting in maximum probability for rejection. Without such specific orientation features, a long particle would orient itself parallel to the lines of flow through the openings, causing high probability for its acceptance -- which is undesirable.
In these conventional screens, high turbulence next to the inlet side of the screen cylinder is, of course, detrimental to both mechanisms cited. In the first, the critical network would be destroyed; in the second, debris orientation would be more random, favoring passage of undesirable particles through the cylinder.
On the other hand, total lack of turbulence or fluid shear is also detrimental in these screens because this condition would allow fibers to form flocs which would be rejected by the screen, causing intolerably high fiber loss.
Conventional screens thus must feature a careful balance between the degree of turbulence and the size of openings employed. Small changes in operating condition can destroy the balance, resulting in either plugging of the screen or in highly-contaminated accepts.
In summary, although prior art screens disclose the use of horizontal slots, rotating foils and flow impediments such as bars fixed to the surface of the screen, and it is mentioned in one patent that vertical slots in a screen could be as small as 0.003 to 0.006 inch in width, as a practical matter it has been found that with prior art screens of this type the slots must be appreciably wider than this, generally wider than 0.010 inch, in order to prevent fractionation and permit sufficient paper making fibers to pass through the screen to provide an economical system.
This means that undesirable material smaller in size than the screen openings can and does pass through the larger openings of prior art screens of this type. Traditionally, therefore, it has been considered necessary that either an appreciable amount of undesirable rejects must be accepted to obtain required quantities of fibers of longer lengths or an appreciable amount of desirable fibers must be lost if smaller rejects are to be screened.